Interleukin-13 (IL-13) is a cytokine secreted by T lymphocytes and mast cells (McKenzie et al. (1993) Proc. Natl. Acad. Sci. USA 90:3735-39; Bost et al. (1996) Immunology 87:663-41). IL-13 shares several biological activities with IL-4. For example, either IL-4 or IL-13 can cause IgE isotype switching in B cells (Tomkinson et al. (2001) J. Immunol. 166:5792-5800). Additionally, increased levels of cell surface CD23 and serum CD23 (sCD23) have been reported in asthmatic patients (Sanchez-Guererro et al. (1994) Allergy 49:587-92; DiLorenzo et al. (1999) Allergy Asthma Proc. 20:119-25). In addition, either IL-4 or IL-13 can upregulate the expression of MHC class II and the low-affinity IgE receptor (CD23) on B cells and monocytes, which results in enhanced antigen presentation and regulated macrophage function (Tomkinson et al., supra). Importantly, either IL-4 or IL-13 can increase the expression of VCAM-1 on endothelial cells, which facilitates preferential recruitment of eosinophils (and T cells) to the airway tissues (Tomkinson et al., supra). Either IL-4 or IL-13 can also increase airway mucus secretion, which can exacerbate airway responsiveness (Tomkinson et al., supra). These observations suggest that although IL-13 is not necessary for, or even capable of, inducing Th2 development, IL-13 may be a key player in the development of airway eosinophilia and AHR (Tomkinson et al., supra; Wills-Karp et al. (1998) Science 282:2258-61).